Retention times, micellar electrokinetic

Lee et al. [30] described a micellar electrokinetic capillary chromatographic method for the determination of some antiepileptics including valproic acid. They used a fused silica capillary column (72 cm x 50 pm) and SDS as the micellar phase and multiwavelength UV detection. Reaction conditions, such as pH and concentration of running buffer were optimized. Solutes were identified by characterizing the sample peak in terms of retention time and absorption spectra. Recoveries were 93-105%. 

The general resolution equation for two neutral analytes with similar retention factors in micellar electrokinetic chromatography, is similar to the relationship for chromatography (section 1.6) with an additional term that arises from the limited migration time window [11,12,166,177,178]. 

As detailed in Chapter 3 by Terabe, micellar electrokinetic chromatography (MEKC) is a useful technique in the retention analysis of water-soluble compounds. The separation and analysis of lypophilic analytes, however, may be difficult in MEKC due to the strong affinity of lypophilic compounds to the micelle resulting in long separation times and poor resolution. An interesting approach for the simultaneous analysis of water- and fat-soluble vitamins by microemulsion electrokinetic chromatography (MEEKC) was proposed by Sanchez. The separation of both water- and fat-soluble vitamins (Bi, B2, B3, Be, B12, C, A palmitate, D, E acetate, and K) was obtained when the microemulsion was prepared with sodium dodecyl sulfate (SDS) as the surfactant, octane as the nonpolar modifier, butanol as the cosurfactant, and propanol as the second cosurfactant. Complete separation of all vitamins was carried out within 55 min however, this approach was tested only in multivitamin formulation. 

Powell, A.C. Sepaniak, M.J. Development of a model for predicting retention times in solvent-gradient micellar electrokinetic capillary chromatography. J. Microcol. Separ. 1990, 2 (6), 278 284. 

The more soluble the neutral molecule is in the micelle, the more time it spends inside the micelle and the longer is its migration time. The nonpolar interior of a sodium dodecyl sulfate micelle dissolves nonpolar solutes best. Polar solutes are not as soluble in the micelles and have a shorter retention time than nonpolar solutes do. Migration times of cations and anions also can be affected by micelles because ions might associate with micelles. Micellar electrokinetic capillary chromatography is truly a form of chromatography because micelles behave like a pseudostationary phase. Solutes partition between the mobile phase (the aqueous solution) and the pseudostationary micelles. 

Separation of antibody-antigen complex from unreacted Ab and Ag has been achieved with a micellar electrokinetic capillary chromatography (MECC) method using cholate micelles and the appropriate buffer pH, temperature, voltage, and electrolyte and modifier concentrations [76], Mixing of antibody and antigen before their injection into the capillary showed that the immuno-complex has a longer retention time than antibody alone. 

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